Some conventional optical systems and methods for accomplishing this task define two nested fields within which imaging is desired: a field of view (“FOV”), the largest scene or area which an optical system can take in all at once; and a so-called “field of regard” (“FOR”) that is considerably larger. The FOR defines an entire region which is to be imaged, but by definition cannot be imaged all at once.
Accordingly the FOV is successively moved around, in the FOR, so as to image portions of the FOR a little at a time—and conventionally this is done by bodily moving the whole optical system. Such motion is achieved by mounting the entire optical system in a gimbal box so that the optics (and the FOV) can be pointed in any direction within the FOR, and operating hand mechanisms or electrical motors to change the pointing direction.
Manual operation is a particularly good solution for price and reliability, and also when it is desirable to direct attention toward scene details whose shape or color is not known in advance, since humans are particularly well adapted to noticing such things. If it is desired to canvass an entire scene very quickly, however, then manual operation of the gimbal box tends to come up short—and greater interest is directed to automatically controlled motors.
With such motors, on the other hand, come many additional undesired elements of extremely great cost and complexity: first, the gimbal box must be provided with fittings, gears etc. for compatibility with automatic operation by the motors; second, the electrical control signals to drive the motors must be somehow programmed to move the gimbal box in such a way as to repetitively scan the FOV within the FOR (for example, but not necessarily, in a raster pattern), missing no part of the FOR; and third, ideally the system is adapted to react when a scene detail of particular interest is noticed—again, either by humans or by automatic recognition equipment.
Once again, human vision is particularly inexpensive and effective when the scene and details of greatest interest are in fact visible; however, this last condition cannot always be guaranteed. In any event the modern desire for rapid imaging and inspection of an entire scene tends to demand automaticity in detail recognition as well as gimbal maneuvering.
Still further in the listing of additional extreme cost and complexity: fourth, the automatic motor control should also be adapted to halt or pause in the repetitive scanning, and direct the FOV specifically toward a particularly interesting scene detail that has been noticed; and fifth, even when that redirection is occurring some sort of ongoing background scan of the entire scene continues to be desirable. Such redirection with ongoing scan, too, can be programmed—but as will be understood such programming is inordinately complex and difficult.
One factor that makes such operation almost completely impractical is the mass and weight of the optical system itself. Automatic control of motors driving a gimbal box that houses an entire optical system calls for powerful, high-torque motors—and therefore high electrical currents, and in turn therefore large-capacity power supplies and finally automatic controllers that are capable of rapidly modulating such high currents from such large power supplies.
Accordingly, even though the technology that has been described might be regarded as relatively primitive, and the available response times from such technology are rather poor, at the same time the cost is all but prohibitive. The gimbal system, with its fine mountings, high-precision motors, extreme power characteristics, and great programming demands, is the primary cost driver and performance limiter for many fine-imaging systems—especially systems requiring a large FOR, and even more so if broad spectral response is also desired.
As already suggested, gimbals furthermore limit system performance. The gimbal system itself must be of relatively massive construction, leading to mechanical resonances that are of low frequency, and these dictate control-electronics bandwidth that is similarly low.
Therefore the gimbals cannot track high-frequency disturbances, even from nearby equipment, and the optics are thereby left vulnerable to jitter—which undesirably smears the image. Gimbals even escalate the size, weight and power requirements for apparatus needed simply to transport the optical system and its gimbal box from one place to another.
The earlier patent documents mentioned above introduce alternatives to gimbals for maneuvering an FOV within a very broad FOR. Certain of these alternatives emphasize devices that are in some ways analogous to the deformable mirrors known in very large astronomical telescopes. Those devices emphasized in the above-mentioned patent documents, however, also have major advantages over such deformable mirrors, as will be seen in a later section of this document.
Historically, a deformable mirror (“DM”) consisted of a monolithic reflective face-sheet that had an array of actuators—which deformed the face-sheet through the application of force normal to the face-sheet, or torque, or both, in order to correct for spurious optical-wavefront irregularities. Thus conventional DMs are particularly intended to achieve very fine diffraction-limited imaging, rather than to maneuver an FOV within an FOR; however, the diffraction limit can be of distinct interest for present purposes as well.
Whether located at a primary mirror or at another optical pupil, as in the active-imaging (“AI”) telescope on Haleakala, conventional DMs have several limitations—discussed at some length in the above-mentioned documents—most particularly minimal ability to direct the sensor FOV within a large FOR.
Also discussed in those documents are some known technologies for wavefront sensing and analysis, used to collect information for operating a DM to improve imaging sharpness. Such sensing techniques generally measure and report modulo 2π/λ phase, with the goal of returning diffraction performance to a theoretical ideal. Four wavefront-sensing methodologies are:                the Shack-Hartmann wavefront sensor, in which a microlens array focuses the wavefront on a focal-plane array, from which a wavefront reconstructor develops a wavefront map that in turn is used to create a DM-actuator command set;        a stochastic-based image-quality sensor, such as developed by Weyrauch et al., that evaluates the system image quality directly for a point source—continually training itself to achieve a desired image metric, such as maximizing or minimizing the intensity of a laser beam that is detected behind a pinhole—by perturbing each element in a DM of the MEMS-array type (see below) and interpreting the resulting influence upon the metric; Weyrauch teaches no technique for wavefront corrections in a system operating over a very wide angular FOR;        somewhat analogously to the Weyrauch work, iteratively minimizing the radius of an image point-spread function (“PSF”)—described in other literature, but apparently not in the environment of a MEMS array;        the pixelated phase-mask dynamic interferometer, which acquires a phase-shifted interferogram in a single camera frame, yielding at each image point a phase difference between two radiation beams;        a shearing wavefront sensor, consisting of a grating in the image plane—where the DM, actually at an image pupil, is reimaged—that has been used for both segment phasing and slope correction for extended complex scenes as well as a point source, and potentially high bandwidth.The Weyrauch development is reported in T. Weyrauch, M. A. Vorontsov, T. G. Bifano, J. A. Hammer, M. Cohen, and G. Cauwenberghs, “Microscale Adaptive Optics Wavefront Control with a Micro-Mirror Array and a VLSI Stochastic Gradient Descent Controller,” 40 App. Opt. 4243-53 (2001).Components not ordinarily associated with high-quality imaging technology—Three kinds of devices are discussed here:        
Microelectromechanical Systems (“MEMS”) Mirror Array
These are the above-mentioned devices which are in some ways analogous to DMs. The first significant commercial use of MEMS mirrors was in the Texas Instruments Digital Light Projector (DLP) MEMS array. Formed in an array of 1 k×1 k two-axis 10 μm mirrors, the bistable mirrors were controlled open-loop, with the mirrors stepped from ±10° locations at rates on the order of 10 μs per step.
Apart from the coowned patent documents mentioned above, to the best of our knowledge the use of MEMS mirrors analogously to DMs has been reported only with wavefront sensors to drive the corrections. In the wavefront-sensor context it is conventionally not shown how to obtain corrections over a very wide angular range of field locations—i.e., within a large FOR.
This limitation is severe if it is desired to provide a system that can monitor a FOR of 60 to 100° or more. Furthermore, even if this limitation were overcome, available MEMS mirrors would not have been adequate to the task.
The DLP mirrors, for example, could only operate in the tip and tilt directions. They were not capable of so-called “piston” movements—i.e., linear motion in or out relative to the overall mirror-array backplane.
Tip and tilt adjustments are desirable for matching wavefront slopes (as well as beam pointing), but piston adjustment too is usually needed to compensate the rectilinear, stepwise profile of a wavefront. In a MEMS array, furthermore, when the mirrors are rotated to point an image or FOV in a particular direction, wavefronts are disturbed by rotation-generated offsets between the planes of adjacent individual mirrors—especially when the offsets are significant in comparison with the wavelength of the incident light.
Furthermore the DLP mirrors were not analog or even multistate binary—i.e., each mirror could take on only one of two positions about each axis. Wavefront correction typically calls for adjustment to rather small fractions of a wavelength; therefore either analog or fine-granularity multilevel binary operation is usually needed.
A more closely related development in MEMS scan-mirror arrays was in the area of optical switching; here the mirrors could be controlled open-loop about one or two axes over the entire range of mirror travel, and thus were “analog” in the sense of being able to point the mirror in generally arbitrary directions.
Examples of this technology include optical switches from Lucent and from Calient Networks. These arrays are typically larger, hundreds of micrometers to ones of millimeters—but have millisecond-level step response characteristics because they are controlled open loop.
Also, areal densities of these arrays are low, less than fifty percent. Therefore significant modifications to their architecture are required to obtain an adequate DM for any sort of adaptive optical system.
Liquid Lenses
The cellular-telephone industry has developed a lens assembly that can actively vary focal length over the angular extent of the FOV, particularly for use in cellular-phone cameras. It is a fluidic-based lens, providing a zoom capability. One such report of interest is B. Hendriks, S. Kuiper, “Variable-focus liquid lens for miniature cameras”, 85 Applied Physics Letters No. 7, August 16.
This development may be useful for present purposes, particularly if an infrared-transmitting fluid can be used. Given the challenge of wavelength specificity as will become clear below, this approach is not viewed as a likely solution.
Spatial Light Modulators (SLMs)
SLMs come in two basic configurations, which very roughly compensate wavefront error by fitting the error through discrete piecewise phase steps. Some SLM arrays consist of mirrors that are adjustable in-and-out, i.e. in piston.
An SLM is not the same thing as a MEMS array, and the two should not be confused. Among several important differences, an SLM moves in piston only, and so can only be adjusted to compensate the stepwise profile, not the continuous slopes, of an incoming light-beam wavefront.
Lucent Technology is understood to be working on a several-million-element SLM device, and Boston Micro Machines has produced smaller ones. While SLMs are certainly part of the DM field, they cannot be associated with DMs of highest precision.
This is because, as noted above, they operate only in piston. Lacking tip and tilt capability, they do a fair job of compensating for wavefront offsets only, in sawtooth fashion (analogous to the so-called “aliasing” in computer images), but not in matching wavefront slopes. Ironically this deficiency is precisely opposite to the above-described limitation of MEMS arrays heretofore—i.e. tip and tilt only, with no piston.
Furthermore even the SLM piston excursion that is available, per actuator, is not high enough to provide the necessary offsets, without an inordinately high actuator density. As will later be seen, the density of actuators required for fully satisfactory wavefront matching, in piston, is almost impractically extreme.